Last year, influenza dinged the U.S. economy for $5.5 billion in lost productivity, hospital visits, and other costs, with the vast majority of that coming from people who didn’t roll their sleeves up for a jab.

In all, vaccine-preventable diseases cost the country about $9 billion in 2015, with 80 percent of that coming from people who failed to get vaccinated, according to a new study in the journal Health Affairs. While close to 17 million influenza cases accounted for the lion’s share of the losses, pneumonia, meningitis, hepatitis B, and HPV infection all had significant parts to play.

From a public health standpoint, this is low-hanging fruit—or should be. Last year, only about 42 percent of U.S. adults over age 18 got a flu vaccine. A separate study recently found that millennials often cited the cost of a flu shot as the reason for not getting vaccinated. But this doesn’t make much sense: buying flu medication once you get sick costs about the same as a vaccine, and missing work is very expensive for anyone who gets paid by the hour. Millennials were also skeptical that the shots would work (they do), as were their older counterparts—49 percent of adults aged 35-44 said they didn’t plan on getting the vaccine.

“Vaccines are one of the safest and most cost-effective health interventions, with numerous social and economic benefits,” the researchers write. Despite that, they say, “adults in the United States are not receiving vaccinations at recommended levels.”

People have never been particularly good at making decisions in their own economic self-interest, but this one seems pretty simple: get your shot, stay healthy.